Field of the Invention
The invention relates in general to the field of torque measurements and, in particular, to a novel approach to the measurement of a torque applied to the cylindrical component of a rotatable object.
Description of the Prior Art
Torque is the measure of the force that causes an object to rotate about an axis. When a tangential force is applied at a point of an object using a lever arm equal to the distance from the axis of rotation, torque is defined as the product of the force times the length of lever arm. In the case of a cylindrical object or mechanical part, such as a shaft rotating in a bearing, the tangential force is applied in some manner on the surface of the part and measured by some means.
Monitoring torque is sometimes critical to the performance of axles, drive trains, gear drives, electric and hydraulic motors, and gas and steam turbines, for example. Therefore, torque measurements are used routinely for quality control in the manufacture of rotating parts in order to ensure that design specifications are met. Such measurements are typically carried out using a strain gauge coupled to the object and to a driving motor. As illustrated schematically in FIG. 1, a test part 10, such as an axle supported by a bearing 12, is coupled by friction or other means axially to a strain gauge 14 through a shaft 16 attached to the gauge. The strain gauge is in turn coupled to a motor 18 through a connecting driving shaft 20. All parts need to be perfectly collinear with respect to the axis of rotation A of the part. As the motor 18 exerts a rotating force on the axle 10 through the shafts and the strain gauge 14, very small deformations within the gauge are detected and measured from which the torque applied to the axle is calculated. Typically, an increasing torque is applied to a stationary part and increased until rotation begins and reaches a constant speed with minimal torque application. A graph of applied torque versus circumferential distance is thus developed for the part so tested.
When very low torque values are measured, such as when a small part rotates within an air bearing driven device, the traditional approach to torque measurements is inadequate for quality-control purposes. For reliable results, the target torque values need to be orders of magnitude greater than any extraneous force affecting the measurement. However, ever present parasitic losses, such as produced by eddy currents in the system, and friction losses in the bearings supporting the mechanism acting on the part may be greater than the actual torque required to rotate the part. In addition, any misalignment along the axis of rotation between the various components of the measurement system (driving motor, shafts, strain gauge, and part) will produce forces or bending of components that will be detected by the strain gauge and mask the actual target torque value.
Another problem with conventional torque measurements of very low values lies in the sensitivity and size of the strain gauges required for such measurements. As the components of the measuring system are reduced in size to accommodate the measurement of very-low-torque parts, the signal-to-noise ratio may decrease beyond acceptable values as a result of the attendant added flexibility of the shaft holding the strain gauge. Under such conditions, the measurement yielded by the system would be uncertain and unreliable for repeatable quality-control purposes.
In view of these problems, prior-art measurement systems are not adequate for measuring parts that require the application of a very low torque to initiate and maintain their rotation. This invention is directed at providing a new approach that overcomes these problems with a mechanism suitable for inline quality-control purposes in manufacturing environments.